Many portable electronic devices nowadays comprise rechargeable batteries, and it is useful, if not imperative, that a user of such a device can readily gauge the amount of power remaining in the battery. Knowing the instantaneous remaining battery capacity helps the user to estimate how long the device can be used, for a given duty cycle, before its battery needs to be recharged.
As such, most battery-powered devices are provided with a “fuel gauge” type indicator, which is commonly in the form of “bars” of a graphical user interface (GUI) element, or a series of LED indicators. By displaying the remaining battery charge, the user can judge whether, and how, to continue using the device.
Existing battery charge level indicators comprise circuitry that interfaces with, and which measures certain parameters of the battery, and a computational element that converts the measurements into a useful indication of the remaining battery power.
The two most commonly used display formats are obtained via a State of Charge (SoC) calculation, which yields either a percentage remaining battery charge, and/or the remaining battery capacity, say, in mAh. In addition, it is also possible to measure the instantaneous, or average, power consumption of the device to yield a “time to fully-discharged” indication in units of time.
Most portable electronic devices are configured to operate differently depending on the battery charge level. In essence, functionality can be reduced as the remaining battery power drops below certain thresholds, such that the device can be fully-operational (i.e. all features operative) above a first certain threshold level, with functionality being withdrawn as the battery charge level decreases. For example, the screen brightness may be dimmed, WiFi disabled, power-hungry applications disabled, etc. to conserver power as the remaining battery power drops below a series of pre-defined threshold values.
Commonly, therefore, the SoC indicator provides a “100%” reading when the battery is fully charged, and a “0%” reading when the battery charge level drops below a voltage cut-off threshold value. Usefully, the voltage cut-off threshold value does not correspond to 0% remaining battery power, to enable certain device functionality, such as a RAM maintenance voltage, an internal clock, the charging control circuitry, and so on, to persist even when the device notionally switched-off. Such a configuration enables the device to operate correctly when it is connected to a charging power source, even though its functionality may be pared-down until the battery charge level exceeds the voltage cut-off threshold value.
The voltage cut-off threshold value depends on the device manufacturer's preferences, although this voltage is usually around 3.2V to 3.0V.
The remaining battery power calculation is typically performed via: a voltage approach, whereby the battery voltage is measured and compared with a value stored in a lookup table of voltages versus internal battery impedances—the intersection yielding a State of Charge or the remaining capacity. Additionally or alternatively, a “coulomb counter” approach can be used, whereby the remaining capacity calculation is based on measuring the current flowing into, and out of, the battery through a sense resistor. Summing of the “in” and “out” currents can give the total charge that has flown from (e.g. by use), or to (e.g. via charging), the battery and the capacity can be then calculated.
The above two techniques are often used both together in proprietary algorithms to provide the best accuracy.
To obtain a SoC estimation using the above “voltage” approach, the measured battery voltage must be corrected by a voltage drop factor as a result of the battery's internal impedance. The known way to measure the internal impedance of the battery is the so-called “relaxation method” as described in, for example, “Battery management systems—Accurate State-of-charge indication for battery-powered applications”, V. Pop and al., Philips Research Book Series, Volume 9, Springer Science—2010 (the disclosure of which is incorporated by reference). This technique involves discharging the battery followed by a rest period, whereby the impedance is calculated by the voltage difference divided by the current load, as set forth in equation 1 below:Zint=(Vrest−Vmin)/ILoad  (Eq. 1)
Where                Zint is the internal battery impedance        Vrest is the battery voltage after a rest        Vmin is the battery voltage loaded by the current ILoad         Iload is the current load applied to discharge the battery for a while (discharge pulse time depends on the number of points to characterize the battery impedance)        
The main problem with the known SoC estimation techniques lies in that the characterization of the impedance of the battery must be performed using specialized laboratory equipment. Because the battery is characterized under laboratory conditions, the characterization might not transpose correctly in actual use, for example when run in an actual application environment. Also, because the battery characterization is carried out independently of system design, the PCB of the hardware and/or the application cannot be taken into account at during characterization, which can lead to inaccuracy in use.
A better approach would be to obtain an impedance reading directly in the application layer, or by the system/device, in use, using real time measurement tools available on the system.
Another drawback of laboratory-based battery characterization subsists in the time it takes to perform the characterization, which can be very long. As described above, the DC impedance can be easily estimated by the by the relaxation method, but this method requires a discharge and a rest time of about 1 hour to be sure that the battery is really in the relaxed phase after stress due to the current load. Therefore, if 100 data points are required, per battery, for the SoC algorithm to work properly, it will take at least 100 hours of characterization time and so the characterization of a battery usually takes around 5 full days to complete.
A need therefore exists for an improved and/or an alternative way to obtain the DC impedance of a rechargeable battery without necessarily having to resort to expensive, and time-consuming, laboratory-based characterization techniques. More specifically, a need exists for a method and/or an apparatus that can obtain the state of charge of a rechargeable battery using battery impedance measurements taken using on-system components.